Fix misplaced guard

This fixes an error in https://codereview.chromium.org/2125333002 (Add choke point for modifying non-AA rect draws (e.g., applying clipping)).

In GrDrawContext::drawRect, when we need to draw a filled rect, we always want to call drawFilledRect when useDrawInsteadOfClear is false. With the buggy code I believe we were falling back to drawing a path.

TBR=bsalomon@google.com
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Review-Url: https://codereview.chromium.org/2135883002